Cathode for electron discharge tubes



v April 29, 1941. w, N U 2,239,954 CATHODE FOR ELECTRON DISCHARGE TUBES Filed June 1, 1939 ELECTRON EMISSIVE SURFACE i ik\ saLvER t-ALKALINE METAL coPPER- SlLVER 0x105 FIG.2

METALUC FOUNDATION FOUNDATION COATED WTTH COPPER LAYER LAYER OF STLVER OF PRE DETERMINED THICKNESS DEPOSiTED ON @OPPER LAYER SiLVEfi LAYER cmwmmaw GXTDTZED SELVER- OXIDE SURFACE ALKMJZED ELEGTRON EMTSSWE CATHOBE INVENTOR FANSELAU Patented Apr. 29, 1941 CATHODE FOR ELECTRON DISCHARGE TUBES Werner Fanselau,

Berlin-Tempelhof, Germany, assignor to Fernseh Akti Zehlendori', Germany engesellschaft, Berlin- Application June 1, 1939, Serial No. 276,795 In Germany June 1, 1938 4 Claims.

My invention relates to a cathode for electron discharge tubes, and particularly to a cathode of the so-called oxide type.

Cathodes are known which consist of a foundation metal, a layer of the oxide of this metal, upon which a suitable metal is deposited. Such cathodes can be produced so as to show good thermionic, photoelectric and secondary emission. As is well known to those skilled in the art, the thickness of the oxide layer is of great importance for obtaining greatest sensitivity and may be difierent for thermionic, photoelectric and secondary. emission.

Hitherto it has been standard practice in the production of some of these cathodes to produce the oxide layer on the foundation metal by subjecting the same to a glow discharge in the presence of oxygen. It has been necessary to exercise extreme care to obtain the critical thickness of the oxide, whereby the change in color of some foundation metals, such as silver, was used as an indication of the depth of the oxide layer produced. In view of the fact that this process is so critical, it has been proposed to, determine the amount of time and current for the glow discharge required to produce a certain oxide thickness and to operate with these values once determined. This method, however, is not sufficiently accurate for reproduction of oxide layers of identical depths.

It is the objectof my invention to provide a cathode, equally as sensitive as cathodes hitherto known, which can be made in a much simpler way; to provide a method by which oxide layers of identical thickness can be readily produced; to provide a new method in which visible observation of the oxide layer during the oxidizing process is not required for determination of the oxide thickness.

Broadly considered, my invention employs a layer of the metal to be oxidized of such predetermined thickness that it will, upon complete oxidation, render an oxide layer of the desired depth. According to my invention the cathode is produced in the following manner: Upon a surface of a foundation of a suitable metal is deposited a thin layer of a metal which oxidizes only superficially in a glow discharge; upon this latter or intermediate layer is deposited a layer of the metal, the oxide of which is desired, of such a thickness that the desired oxide thickness is obtained after complete oxidation.

The active emitting material is then deposited upon the oxide layer in a manner well known to those skilled in the art.

The invention will now be explained more in detail in connection with the production of a caesiated photoelectric cathode according to my novel method. u

I prefer to use silver as the foundation or base metal. Upon the surface intended to act as the emitter Iprefer to deposit a thin layer of copper of a thickness in the order of, say, between 0.5 and 10 microns. The deposit of the copper can be accomplished, for instance, in an electrolytic bath or by evaporation, as may seem preferable. A layer of silver is then deposited by electrolysis or evaporation upon the copper surface, whereby the thickness of the silver deposit can be readily predetermined to a high degree of accuracy by using the proper concentration of the solution, the proper amount of current flow and proper duration of the process in the case of an electrolytic deposit, or' by predetermlning the proper amount of silver in the case of evaporation. The proper values can also be readily determined by experiment. The next step is to oxidize this thin deposit of silver, preferably in a glow discharge in the presence of oxygen. The oxidizing process is not critical 'at all provided only that it secures complete oxidation of the thin silver deposit, because oxidation stops at the copper surface, which oxidizes only superficially. The silver oxide surface is then caesiated in accordance with one of the various methods well known in the art. Such a cathode is shown in cross section in Fig. 1 of the drawing, and the process is illustrated in Fig. 2.

Thus I have provided a novel type of cathode of the so-called oxide type and a simple and efficient method of producing the same, and while I have described my method in connection with a cathode of the type containing silver, silver oxide and caesium, as are preferred for photoelectric emission, I do not wish to be limited to such cathodes, inasmuch as all cathodes in which an oxide layer is produced in accordance with the 2 Q t aesaees above, teachings are deemed to be well within the scope of the present invention.

What I claim is:

1. The process of making cathodes of the oxide type, which comprises the steps of providing a surface of a metallic foundation with a coating of a metal which oxidizes only superficially in a gloa discharge in the presence of oxygen, depositing upon said coating a layer of predetermined thickness of a metal, completely oxidizing said lastnamed layer in a glow discharge in the presence of oxygen to convert said layer or metal into a layer of oxide of predetermined thicmess, and all-ralia ing said layer of oxide to produce an electronemissive cathode.

2. The process of making cathodes of the oxide type, which comprises the steps of providing a surface of a metallic foundation with a coating of a metal which oxidizes only superficially in a glow discharge in the presence of oxygen, depositing upon said coating a layer of predetermined thickness of the same metal as used for said foundation, completely oxidizing said lest-named layer in a glow discharge in the presence of oxygen to convert said layer of metal into a layer of oxide of predetermined thickness, and alkalizing said layer of oxide to produce an electronemissive cathode.

3. The process of making cathodes of the oxide type, which comprises the steps of providing a surface of a silver foundation with a coating of copper, depositing upon said copper coating a layer of sliver of predetermined thickness, completely oxidizing said last-named silver layerin a glow discharge in the presence of oxygen to convertseid layer of metal into a layer of oxide of predetermined thickness, and alkallzing said layer of oxide to produce an electron-emissive cathode.

d. The process of making cathodes of the oxide type, which comprises the steps of depositing a layer of metal of predetermined thickness on a surface of a m tallic foundation which oxidizes only superficia ly in a glow discharge in the 20 presenceof oxygen, completely oxidizing said layer 

